1. Field of the Invention
The present invention relates generally to optical systems for image viewing. More particularly, the present invention relates to an optical system which utilizes a pair of ellipsoidal reflective surfaces to provide for wide field of view imaging.
2. Description of the Prior Art
Currently, wide field of imaging optics are used for a multitude of purposes including photographic, remote sensing and space surveillance imaging. Generally, wide field of view imaging systems are refractive or catadioptric, and thus suffer from chromatic aberration and are generally to heavy for applications such as space surveillance.
Reflective optics do not suffer the drawbacks of refractive optics. Wide field of view imaging systems, however, share a common problem which is field curvature. This prevents a user from having the image sensed by a flat detector such as focal plane array or photographic film. In addition existing reflective wide field of view imaging systems are complex in nature requiring multiple mirrors to implement the system.
An example of a prior art wide field of view optical system is disclosed in U.S. Pat. No. 4,566,763 for a xe2x80x9cPanoramic Imaging Block For Three-Dimensional Spacexe2x80x9d, which issued Jan. 26, 1986. Disclosed in U.S. Pat. No. 4,566,763 is a wide field of view optical system which is suitable for pictorial recording and displaying based on a flat cylindrical perspective and reflective and refractive surfaces.
Another example of a prior art wide field of view optical system is disclosed in U.S. Pat. No. 4,037,943 for a xe2x80x9cReflection Type Image Forming Optical System Having A Large Angle Of Viewxe2x80x9d. Disclosed in U.S. Pat. No. 4,037,943 is a reflective type image forming optical system having a large angle of view which includes a convex mirror for reflecting incident light and a concave mirror. The convex mirror is disposed with its reflecting surface facing the reflective surface of the convex mirror. The concave mirror is arranged so that the distance between the center of curvature of the convex mirror and the center of curvature of the concave mirror is greater than half the radius of curvature of the concave mirror. There is also a stop disposed between the reflecting surfaces of the convex and concave mirrors.
While the foregoing prior art wide field of view optical systems are fairly effective, there are still drawbacks with these systems including chromatic aberration, weight problems and complexity in design.
Accordingly, there is a need for a relatively simple in design optical system which uses less than three mirrors to provide a high resolution wide field of view. In addition, the optical system should provide for a means whereby an image sensed by a flat detector such as focal plane array or photographic film is recorded by the flat detector.
The present invention overcomes some of the disadvantages of the prior art including those mentioned above in that it comprises a relatively simple optical system which includes a pair of mirror for receiving light from an object and directing the light onto a flat detector to record a wide field of view image of the object on the flat detector. The flat detector may be, for example, a photographic film.
The optical system of the present invention is a two reflective surface optical system for imaging a wide field of view scene and recording the scene on a flat detector. The optical system utilizes a pair of ellipsoidal reflective surfaces or mirrors with different eccentricities such that the ellipsoidal reflective surfaces have shared foci or foci in close proximity to one another. Light from a wide field of view scene is reflected from the reflective surface of the first ellipsoid forming a virtual image near one of the shared foci. The reflected light is then directed toward the reflective surface of the second ellipsoid and is reflected from this surface forming a real image near the other shared foci. The elliptical shape of the second reflective surface directs light from the virtual image near one of the shared foci directing the light near the other shared foci.
In the optical system of the present invention, the first reflective surface is convex and the second reflective surface is concave.